1. Field of the Invention:
The present invention is directed to a crankshaft which is supported by five supports and has eight balance weights for an eight-cylinder internal combustion engine, wherein the eight cylinders comprise two sets of four cylinders which define a V-angle of 90.degree. therebetween (which will be called a 90.degree. V-type eight-cylinder internal combustion engine hereinafter) and, more particularly, is directed to relationships in arrangement and in weight between the eight balance weights of the crankshaft.
2. Description of Related Art:
Relationships in arrangement between balance weights of a prior art crankshaft for a 90.degree. V-type internal combustion engine is disclosed in Japanese Utility Model Publication SHO 47-29106 and Japanese Patent Publication SHO 59-27463.
FIG. 11 illustrates the arrangement of the eight balance weights disclosed in Japanese Utility Model Publication SHO 47-29106 where each balance weight 201, 202, 203, 204, 205, 206 is arranged in a 0.degree. or 90.degree. position as viewed from the fore end of the crankshaft.
FIG. 10 illustrates the arrangement of the balance weights disclosed in Japanese Patent Publication SHO 59-27463 where the No. 1 through No. 4 balance weights 101, 102, 103 and 104 are offset by .theta..sub.0 .degree. in the counter clockwise direction from the No. 4 pin 114 positioned at the lowermost position as viewed from the fore end of the crankshaft and the No. 5 through No. 8 balance weights 105, 106, 107 and 108 are offset by .theta..sub.0 .degree. in the counter clockwise direction from the No. 1 pin 111 positioned at the uppermost position as viewed from the fore end of the crankshaft.
Generally, for the purpose of making balance weights as small as possible, it is theoretically known that the balance weights of a crankshaft having eight balance weights should be arranged as shown in FIG. 8 so as to eliminate the primary couple of inertial forces. In FIG. 8, the No. 1 through No. 4 balance weights 1, 2, 3 and 4 are offset by .theta..sub.0 .degree. (.theta..sub.0 .degree. is about 20.degree.) in the clockwise direction from the No. 4 pin 14 which connects the No. 7 and No. 8 balance weights and which is positioned at the lowermost position as viewed from the fore end of the crankshaft, and the No. 5 through No. 8 balance weights 5, 6, 7 and 8 are offset by .theta..sub.0 .degree. in the clockwise direction from the No. 1 pin 11 which connects the No. 1 and No. 2 balance weights and which is positioned at the uppermost position as viewed from the fore end of the crankshaft. Further, No. 2 and No. 3 pins 12 and 13 (not shown) are positioned at 90.degree. in the clockwise direction from the No. 8 and No. 1 balance weights, respectively. Though these arrangements of FIG. 8 are different from those of Japanese Patent Publication SHO 59-27463 in direction and amount of offsetting of the balance weights, the technical idea included therein is similar to that of Japanese Patent Publication SHO 59-7463.
However, in the arrangement of FIG. 8, there is a drawback that the bearing loads of the No. 2 journal connecting the No. 2 and No. 3 balance weights and the No. 4 journal connecting the No. 6 and No. 7 balance weights become as large as about twice those of the No. 1, (No. 3,) and No. 5 journals and there is a fear that sticking may occur at the No. 2 and No. 4 journals. Such sticking will occur in the balance weight arrangement of Japanese Patent Publication SHO 59-27463.
For the purpose of making bearing loads small by making the loads uniform, there is a necessity of making a crankshaft assembly (including a crankshaft, a piston, a connecting rod, a bearing metal and a piston pin) itself highly balanced. Balance weights are provided to eliminate a couple of inertial forces which are generated due to a rotational mass and a reciprocational mass. Therefore, for the purpose of balancing a crankshaft assembly, it is effective that the balance weights are arranged at the positions just opposite the pistons as shown in FIG. 9. This arrangement of the balance weights of FIG. 9 correspond to that of FIG. 11 of Japanese Utility Model Publication SHO 47-29106.
However, in the arrangement of balance weights of FIG. 9, it is required to make the mass weight of each of the No. 1 and No. 8 balance weights greater than any one of the No. 2 through No. 7 balance weights to obtain effective balancing. This results in over balancing of the crankshaft assembly in the directions of the No. 1 and No. 4 pins as well as an excessive increase in the weight of the crankshaft.